Functions

Functions are code blocks that can be called from other parts of your program. You might want to use a function if you are going to use the same code multiple times or if you want to break down a large program into smaller parts.

We have already seen some functions in the previous chapters, like the built-in print function.

print("Hello World")

Creating Functions

The def keyword is used to specify a function followed by the name. Any arguments that the function takes are defined inside parentheses.

When the function is called, the code block is executed much like an if statement.

def hello():
    print("Hello World")

hello() # Output: Hello World

Arguments

Arguments can be passed to a function by specifying them in parentheses. The arguments are variables that can be used inside the function.

Positional Arguments

Positional arguments are arguments that are passed to the function in the same order as they are defined. Each argument is separated by a comma. You can have any number of positional arguments.

def hello(first, last):
    print("Hello", first, last)

hello("john", "doe") # Output: Hello john doe

Exercise

Write a function named add that takes two arguments and prints their sum.

Keyword Arguments

Keyword arguments are arguments that are passed to the function with a keyword. You can even specify positional arguments using keywords.

def hello(first, last):
    print("Hello", first, last)

hello(last="doe", first="john") # Output: Hello john doe

However, keywords are mostly used for optional arguments that have default values. You can also use keywords to specify arguments in a different order than they are defined.

def hello(first, last="doe"):
    print("Hello", first, last)

hello("john") # Output: Hello john doe
hello("john", "smith") # Output: Hello john smith
hello(last="deer", first="john") # Output: Hello john deer
hello(first="jim") # Output: Hello jim doe

If you want to only allow positional or keyword arguments you can use the / and * symbols to specify where the positional and keyword arguments start.

def test(position_only, /, position_or_keyword, *, keyword_only): 
    # code block

The print function is also a good example of a function that takes arguments.

print("Hello", "World", sep="-", end="!\n") # Output: Hello-World!

The first two arguments are positional arguments and the last two are keyword arguments. The print function can take any number of positional arguments, but there are only a few keyword arguments. Here is what the help function says about the print function.

print(*args, sep=' ', end='\n', file=None, flush=False)
    Prints the values to a stream, or to sys.stdout by default.
    
    sep
      string inserted between values, default a space.
    end
      string appended after the last value, default a newline.
    file
      a file-like object (stream); defaults to the current sys.stdout.
    flush
      whether to forcibly flush the stream.

Keyword arguments can have default values so you don't have to specify them every time you call the function. For example, the print function has a default value for the end keyword argument which will add a newline character at the end, but you can override it by passing a different value.

Exercise

Modify the code below to print 0-9 on a single line.

Return Values

Functions can produce a value by using the return keyword. This is useful for creating a reusable expression that can be used in other parts of your program.

def isPositive(x):
    return x > 0

print(isPositive(5)) # Output: True
print(isPositive(-5)) # Output: False

if isPositive(5):
    print("Positive")
else:
    print("Negative")

Exercise

Write a function named add that takes two arguments and returns their sum.

Scope

Scope refers to where a variable can be accessed. There are two types of scope: global and local.

Global scope

A variable created outside of a function has a global scope and can be accessed inside or outside of a function.

def print_x():
    print(x)

x = 10
print_x() # Output: 10
x = 20
print_x() # Output: 20

Local scope

A variable created inside a function is only available inside that function. This is called the scope of the variable. I variables with the same name can exist inside and outside of a function.

x = 5
def print_x():
    x = 10
    print(x)

print_x() # Output: 10
print(x) # Output: 5

Global keyword

If you want to modify a global variable inside a function, you can use the global keyword.

x = 5
def print_x():
    global x
    x += 1
    print(x)

print_x() # Output: 6
print(x) # Output: 6

Exercise

Given a number x, do the following:

1. Create a function called add_one that increases the value of x by 1.
2. Create a function called add_two that takes x as an argument and returns the value of x plus 2.